(Reuters Health) - - As long-distance races like half-marathons and full marathons become more popular, race organizers and medical directors should consider using online medical screening to identify risky runners and avert medical emergencies, researchers say.

After a four-year trial of online pre-race screening and education of non-professional runners in South Africa, rates of overall medical encounters dropped by a third and serious life-threatening events dropped by two-thirds compared to the prior four years, the study team reports in the British Journal of Sports Medicine.

“Exercise has major health benefits to everyone, but as these mass community-based sports events such as the Boston Marathon grow, serious health events do occur that we’d like to prevent,” said lead author Dr. Martin Schwellnus of the University of Pretoria and the International Olympic Committee Research Center in South Africa.

Overall, medical emergencies are rare even in amateur races. The risk of serious medical emergencies during marathons is about 0.5 per 1,000 racers, and rates of sudden death vary between 0.004 and 0.033 per 1,000 racers, the authors note.

“This shouldn’t detract from the overall message that regular exercise is one of the most important lifestyle choices you can make to reduce disease,” he told Reuters Health by phone. “But doing it safely, especially if it’s high intensity and prolonged, could include having a medical check beforehand.”

Schwellnus and colleagues tracked more than 87,000 individual runners over eight years of annual Two Oceans Marathon races in South Africa, tallying the rates of life-threatening and serious, but not minor, medical encounters during races. The series includes a 21k, or half-marathon, race, as well as a 56K, or full marathon, and about 22,000 of the unique runners participated in races during both of the study periods.

During the final four years of observation, researchers introduced an online pre-race medical screening for all race registrants that asked about existing heart disease, heart risk factors such as diabetes and hypertension, prescription medications and chronic diseases including allergies; respiratory, nervous system and immune system disorders; and cancer.

After the screening, all runners received an email with information about potential medical complications that could occur during high-intensity exercise. Based on the screening responses, the researchers also categorized runners into four levels of risk, and those in the top two categories, “high risk” and “very high risk,” were further advised to seek clearance from their doctors before doing the race. No one was prevented from running by the race organizers or the race medical team, however. The final decision was left to the runner.

During the four-year period that included online screening, serious medical encounters dropped by 29 percent compared with the four-year period before screening began. For half-marathons, the rate of serious medical encounters fell by 19 percent and for full marathons by 39 percent. In addition, serious life-threatening events dropped by 64 percent, from 39 events in the first four-year period to 19 events after screening began.

The screening program did not seem to put runners off, the authors note. Over the eight-year period, the number of registered runners grew, from a total of 81,000 in 2008-2011 to 106,000 in 2012-2015. After screening began, however, the researchers saw a significant drop each year in the proportion of runners in the “high risk” and “very high risk” categories.

“People with chronic diseases and who use certain prescription medications should talk to their doctors before starting long-distance exercise,” Schwellnus said. “Those who are starting a program for the first time should plan a gradual progression to the full distance.”

Schwellnus and colleagues are beginning to work with international sports federations for running and cycling to implement a standard screening.

“Whether someone is at low risk or along the gradient of potentially having a health event, it’s good to be aware of the state of your health,” said Jack Goodman of the University of Toronto Goldring Center for High Performance Sport, who wasn’t involved in the study.

“Listen to your body, and any symptoms such as fainting, dizziness and feeling chronically unwell during exercise should be followed up,” he told Reuters Health by phone. “Not everyone is constructed to be engaged in intensive exercise. Diagnosis doesn’t mean the end of participation, rather the right kind of guidance about next steps for exercise.”